Why is it important to enable domain before using lightning?

Hi Anurag,

There are a number of reasons that My Domain is required. The one that will make the most sense to you is with respect to Lightning Components. Every Lightning Component has a "namespace". If you use any managed package components or 3rd party apps, they always have their own "namespace" which is a reference to a custom domain. This allows Salesforce to differentiate between your code and the code from the managed package or components that are running in your org.

Your Lightning Components can't share the same memory space as Lightning Components from a vendor. By having My Domain enabled, they can more readily keep them separate. Previously with Visualforce, this wasn't an issue since pretty much everything ran on the Server side in Apex through a Server Side controller, not on the client side like Lightning does.

Hope this helps.